This invention relates to photodiodes and, more particularly, to a method of fabricating n.sup.+ -p-.pi.-p.sup.+ silicon avalanche photodiodes or n.sup.+ -.pi.-p.sup.+ silicon photodiodes of the p-i-n type.
The advent of the laser and its promise as a carrier source for optical communications has stimulated widespread interest in the development of photodetectors with high sensitivity to weak signals and fast response to light intensity modulations. An optical receiver, which usually includes a photodetector and an amplifier at its output, should satisfy certain general performance criteria according to H. Melchior, J. of Luminescence, Vol. 7, pp. 390-414 (1973): (1) large response (quantum efficiency) at the wavelength of the incident optical signal; (2) sufficient electrical bandwidth (i.e., speed of response) to accommodate the information bandwidth; and (3) minimum excess noise introduced by the detection and amplification process.
One common type of photodetector is the photodiode which contains as an essential element a depleted semiconductor region with a high electric field that serves to separate electron-hole pairs photoexcited through band-to-band excitation. High speed photodiodes are usually connected to a relatively low impedance so as to allow the photoexcited carriers to induce a photocurrent in the load circuit while they are moving through the high field region. Photodiodes for detecting visible and near infrared radition are commonly operated at relatively large reverse bias voltages in order to reduce carrier drift time and lower the diode capacitance without introducing excessively large dark currents (Melchoir supra at 397). In a reversed biased p-i-n photodiode, for example, with radiation incident on the p-layer, the radiation which is not reflected at the surface penetrates some distance into the photodiode material before it is absorbed and generates photocarriers. Electrons and holes generated within the high field region of the junction (i-layer), and minority carriers which diffuse from the p- and n-layers to the junction before recombination, are collected across the high field region and contribute to the photocurrent.
The actual quantum efficiency and speed of response of photodiodes depends strongly on the wavelength of operation and the diode material and design. Silicon photodiodes, for instance, are preferably used in the near ultraviolet and in the infrared up to about 1 .mu.m. But, because of its strongly varying light penetration depth, silicon photodiodes have to be optimized for each wavelength of interest (Melchior supra at 400). The speed of response is reduced at the longer wavelengths as the width of the high field region increases.
Dark currents in photodiodes limit the sensitivity to weak light signals and can originate either from the bulk or from the surface. Surface leakage currents, which are a problem especially in high resistivity silicon photodiodes, can be reduced by special surface treatments and various guard ring structures. On the other hand, bulk leakage currents in silicon photodiodes are mainly due to carrier generation within the space charge layer. For carefully processed silicon diodes dark currents as low as 10.sup.-6 -10.sup.-8 A/mm.sup.3 of depleted volume have been attained (Melchior supra at 405).
One particularly useful type of photodiode is the avalanche photodiode (APD) which combines the detection of optical signals with internal amplification of the photocurrent. Internal current gain takes place in an APD when carriers gain sufficient energy by moving through a high field region of a highly reverse biased junction to release new electron-hole pairs via the mechanism of impact ionization. The current gain of an APD fluctuates due to the statistical nature of the carrier multiplication process. Even for spatially uniform avalanche regions the statistical gain variations give rise to noise in excess of multiplied shot noise and is usually characterized in terms of an excess noise factor given by: ##EQU1## where &lt;i.sub.M.sup.2 &gt; is the mean square noise current at the output of the APD divided by the mean square noise &lt;i.sub.ph.sup.2 &gt; of the primary photocurrent multiplied by the square of the average gain M. In a silicon APD the ionization rate .alpha. of electrons is much larger than the ionization rate .beta. for holes (e.g., .beta./.alpha. = 0.02 to 0.2) and, as a consequence, F(M) increases much more rapidly for hole injection than for electron injection (Melchior supra at 409). This consideration suggests that a silicon n.sup.+ -p-.pi.-p.sup.+ APD be back illuminated (i.e., light made incident on the p.sup.+ -layer remote from the junction so that electrons are injected into the multiplication region) rather than front illuminated (i.e., light made incident on an n.sup.+ -layer near the junction so that holes are also injected into the multiplication region). H. W. Ruegg illustrates the application of this principle in the design of an n.sup.+-p-.pi.-p.sup.+ silicon APD described in IEEE Transactions on Electron Devices, Vol. ED-14, No. 5, pp. 239-251 (1967). In this type of APD, which is particularly useful for high speed detection at GaAs laser wavelengths, carrier multiplication is constrained to the narrow n.sup.+ -p region and the wider .pi.-region acts mainly as a collector for photoexcited electrons generated by light made incident on the p.sup.+ -layer. Ruegg (at 247, column 1) points out that "the optimized device requires the illuminated surface [the p.sup.+ -layer] to be opposite to the p-n.sup.+ junction in order to assure pure electron injection into the multiplication region." Consequently, he requires that the "total device thickness in this case must be of the order of the penetration depth of the light to be detected" (20-30 .mu.m for GaAs laser wavelengths). He goes on to add that "[s]ince wafers of this thickness cannot be handled, the only obvious solution was to etch local cavities (at the sites of devices) into a considerably thicker silicon wafer." Unfortunately, the need to etch uniformly thick cavities, or equivalently to thin the substrate by lapping, increases substantially the cost of manufacturing this type of APD. Cost increases also result because the thinned wafers are difficult to handle, are easily broken, tend to warp making mask alignment difficult, and present difficulty in packaging.
One alternative, therefore, is to form the structure on a thick p.sup.+ region and to use front-illumination through the n.sup.+ -layer so that the wafer does not have to be thinned. But, as mentioned previously, mixed hole and electron injection occurs with an attendant increase in noise. In prior art front-illuminated n.sup.+ -p-.pi.-p.sup.+ silicon APDs (e.g., U.S. Pat. No. 3,886,579 granted to Ohuchi et al. on May 27, 1975) the structures have not been optimized to reduce excess noise, to produce low leakage currents and to be reliable in the long term. It is questionable, therefore, whether such devices are useful in an optical communication system in which receiver sensitivity is typically -55 dBm (e.g., at a 0.825 .mu.m wavelength and a 44.7 megabit/sec data rate). Naturally, then, one would like to reduce the noise penalty and dark currents in a reliable APD without employing unduly complex processing so that the generally lower cost and case of handling advantages of a front-illuminated APD can be exploited.
It is, therefore, a broad object of our invention to fabricate silicon photodiodes such as n.sup.+ -p.pi.-p.sup.+ APDs and n.sup.+ -.pi.-p.sup.+ type p-i-n diodes.
It is another object of our invention to fabricate such an APD which can be front-illuminated without paying an excessive noise penalty.
It is yet another object of our invention to fabricate such a front-illuminated APD which is relatively easy to handle during processing and which is relatively inexpensive to fabricate.
It is one more object of our invention to fabricate such an APD which has low excess noise.
It is still another object of our invention to fabricate such an APD or p-i-n photodiode which also has high quantum efficiency, a short response time, low dark currents, and good reliability.
It is another object of our invention to fabricate such a p-i-n photodiode which has low capacitance and can operate at low voltages.